1. Field of the Invention
The present invention relates to a technique and apparatus for growing single crystal materials such as silicon using controlled microwave heating to establish a step-wise reduced temperature gradient for the silicon being pulled from the melt, while maintaining minimal lateral temperature gradient at each step. This control of the thermal gradients during crystal pulling increases the speed at which silicon can be grown, and enables the growth of silicon in rods having rectangular and other cross-sectional geometries not possible in the prior art.
2. Description of the Prior Art
A substantial percentage of all device grade, single crystal silicon and other semiconductor material presently is grown using the Czochralski method. Silicon is heated to the molten state in a crucible. A single crystal seed is dipped into the melt. As silicon starts to grow on the seed, the crystal is slowly withdrawn from the melt. An appropriate mechanism thereafter pulls the crystal at a rate commensurate with single crystal growth.
As described in the text entitled "Silicon Semiconductor Technology" by W. P. Runyan, McGraw-Hill Book Company, 1965, in Chapter 4, entitled "Crystal Growth", there are two possible rate limiting steps in growth from the melt. One is the time required for atoms within the liquid to diffuse to an appropriate crystal site. This time is usually very short, and in most systems is not the mechanism which limits growth rate. The second rate limiter, and the one normally of most importance, is the requirement that the latent heat of fusion be removed from the crystal-melt system. There are in principle two ways in which this heat may be removed: (a) it may be removed through the body of the crystal itself, or (b) the liquid near the liquid-solid interface may be super-cooled by a heat sink other than the crystal. Because of the high probability of spurious nucleation at the highly super-cooled liquid-container boundary, the latter technique is seldom used.
In most prior art silicon growth techniques, the latent heat of fusion is removed by thermal radiation from the crystal being grown. Heat removal may be aided by providing a heat transfer medium external to the rod. Normally this results in a rod shape with a circular cross-section. Uniform radiation occurs around the entire crystal periphery. At any particular distance from the melt, the rod exhibits a significant lateral thermal gradient in which the temperature is a maximum at the axis and decreases linearly with increasing radius.
There are several consequences of this cooling by thermal radiation from the surface. Although uniform single crystal growth is accomplished, the geometry is limited to a rod shape of circular cross-section. Moreover, the growth rate (i.e., the rate at which the crystal can be pulled while still ensuring single crystal formation) is inversely proportional to the crystal diameter. An increase in growth rate necessitates a reduction in the crystal rod diameter.
One object of the present invention is to provide a silicon growth apparatus and technique which facilitates increase growth rate without a reduction in the rod diameter. Another objective is to provide an apparatus and method for growing single crystal silicon and other material in rods having cross-sectional shapes that are non-circular, including rectangular geometries.
With regard to shape, for some applications, the circular semiconductor shape is economically disadvantageous. An example is the production of solar cells. If formed on circular wafers and mounted next to one another on a planar surface, significant gaps exist between adjacent solar cells. The packing density, and hence the electrical output per square unit of surface, could be substantially improved by using square or rectangular shaped solar cells. However, if such solar cells are cut from circular wafers, a great deal of semiconductor material is wasted. Thus for optimum cost effectiveness, it would be ideal to have the solar cells fabricated from ingots of single crystal silicon which is grown with a square or rectangular cross-section. The present invention facilitates such growth.
Certain techniques have been proposed in the past to control the cooling of the single crystal rod being pulled from the melt. Some of these are reviewed in the book entitled "Semiconductor Crystal Manufacture" by Marshall Sittig, 1969, Noyes Development Corporation. In the U.S. Pat. No. 3,226,203 to Rummel, the longitudinally growing crystal is pulled through a cooling chamber. There coolant oil is circulated to aid thermal transfer away from the crystal. In the U.S. Pat. No. 3,251,665 to Bennett, a thermal gradient control member formed of electrical conducting material and disposed on top of the melt crucible is used to control the thermal gradient at the top of the molten pool in the crucible. In the U.S. Pat. No. 3,173,765 to Gobat et al, the crystal is pulled through a series of chambers having controlled atmospheres. None of these techniques eliminate the lateral thermal gradient in the crystal rod being pulled, and all depend on uniform radiation from the rod surface to cool the crystal.
By contradistinction, an object of the present invention is to provide a technique in which the single crystal material being pulled is actually heated in a controlled manner so as to minimize the lateral thermal gradient at each distance from the melt. A step-wise reduction in temperature is achieved within increasing distance. As a result, removal of the latent heat of fusion no longer depends only on the uniform radiation from the surface of the circular rod. Arbitrary selection of geometry is permissible, and growth rates substantially faster than those of the prior art are facilitated.